BACKGROUND 1. Field of the Invention
This invention relates to the field of electromagnetic field measurement devices, and in particular to devices for underground line location.
2. Discussion of Related Art
It is often necessary to locate buried lines, which are employed by numerous utility companies, in order to repair them, replace them or mark them to prevent their damage during excavation nearby. Examples of buried lines include pipelines for water, gas or sewage and cables for telephone, electrical power or cable television. Many of the lines are conductors, such as metallic pipelines or cables. In other applications, it is often useful to locate lines, such as power lines, that are concealed in the walls of buildings. It is well known to locate concealed lines by passing electrical current through them and detecting electromagnetic emissions that then emanate from them thereby.
A conducting conduit (a line) may be caused to radiate electromagnetically by being directly connected to an external transmitter or by being inductively coupled to an external transmitter. In some instances, such as with power lines, the line may radiate without an external transmitter.
A line locator detects the electromagnetic radiation emanating from the line. Early line locators included a single sensor that detects a maximum signal or a minimum signal, depending on the orientation of the sensor, when the line locator is passed over the line. Later line locators have included two or more sensors to provide information regarding proximity to the line.
Some line locators include two detectors oriented to measure magnetic fields in the horizontal direction (i.e., parallel with the surface of the earth) and arranged along a vertical axis. Typically, signals from these two detectors can be utilized to calculate the depth of the line. These line locators, then, detect the magnetic fields from the line to be located and display to an operator information about the location and/or depth of the line. A method of checking for a distorted field with such a locator is to make two measurements of the field while varying the height of the locator and compare the result. The first measurement is made with the locator at ground level. The second measurement is made with the locator at some given distance, for example six inches, above the ground. If the first measurement does not equal the second measurement plus the distance between the two measurements (e.g., six inches), then it is assumed that there is field distortion present and the measurement of depth is assumed to be suspect. Typically, methods of locating a line and determining its depth depend on the assumption that there is a single line of current along the line, that there are no other sources of electromagnetic fields, and that all responses are linear.
Problems in the depth measurements can arise when lines, other than the line being detected, interfere with the electromagnetic fields radiated by the target line. For example, other lines may become electrically coupled to the line being detected, either directly or inductively, and re-radiate unwanted electromagnetic fields. There may also be other conditions, which cause other electromagnetic fields, not originating from the line being measured, to be present in the location area such as anomalous soil conditions, metal structures, or ground water. These interfering fields or distortion of the magnetic field from the line being measured cause the line locator to incorrectly calculate the depth of the line. Since there is typically no indication of problems associated with the measurement, the operator may erroneously report the depth of the line and therefore either fail to locate the line or hit the line at too shallow a depth.
An incorrect measurement or a measurement on the wrong line can result in injury or damage. If a live power line is dug up by mistake, personnel can be injured and the line and equipment damaged. If a water line is dug into by mistake, the line can be damaged, and water leaking from the line can further cause damage. If a gas line, for example, is damaged during adjacent excavation, injury to persons and damage to property can occur.
Therefore, there is a need for a line locator capable of measuring the position and depth of a line and also of providing an indication of the validity of the measurement. The incorporation of such a feature in a locator could prevent harmful and costly damage to buried lines during nearby excavation.
According to the present invention, a line locator receiver determines whether a detected electromagnetic field is distorted or not. Distortion can be due to other lines in the ground, power sources, or other anomalies.
The electromagnetic field is measured at at-least three different points in space. In some embodiments this is accomplished using a line locator receiver that has three or more detectors at three or more different locations. In some embodiments making depth measurements, the detectors are spaced vertically. However, several detectors at several different positions may be used to make several measurements of an electromagnetic field.
Based on the measurements of the magnetic field at the detectors and a model of an expected field, an error term can be calculated from the measurements and compared to a threshold value. If the error term is larger than the threshold value, a warning is communicated to the operator, indicating that an unacceptable distortion of the magnetic field has been detected.
In some embodiments, many field measurements are made and mathematically processed to provide detailed information about the detected field. Based on the measurements and/or analysis, an operator can determine if a given depth or position measurement is likely to be accurate.
Therefore, by making and processing multiple field measurements, mistakes as to the position, nature, and depth of underground lines can be avoided, thus increasing safety and lowering the risk of damage.
These and other embodiments are further discussed below with respect to the following claims.